1. Field of the Invention
The present invention relates to an optical semiconductor device using an optical semiconductor chip such as an LED lamp. More particularly, it has an object to provide a structure capable of preventing a semiconductor chip to be damaged from variations in environmental temperature to improve reliability and preventing performance to be deteriorated due to the improved reliability to allow the reliability and the performance to exist together.
2. Description of the Related Art
When an improvement of reliability is intended in a conventional optical semiconductor device, an optical semiconductor chip is sealed in a first, relatively soft resinous member to improve resistances against shock and moisture. Further, the first resinous member is sealed in a second, relatively hard resinous member to improve the mechanical strength. In this way, two resins with different characteristics can be employed together to compensate for a function lacked in one from another to improve the reliability. (See Patent Publication 1: JP-A-05/327029, paragraphs 5–7, FIG. 1, for example).
In the above conventional structure, however, on selection of two resins as described in the above example, the relatively soft resin is commonly selected as the inner resin that directly contacts the optical semiconductor chip and the relatively hard resin as the outer resin to achieve the required mechanical strength.
In such the case, a difference occurs in thermal expansion coefficient between the soft resin and the hard resin. The difference may reach 10 times depending on cases. Under a high-temperature ambient, the soft resin surrounded in the hard resin expands in volume and applies an excessive compressive pressure to the optical semiconductor chip. Therefore, a problem is caused because such the pressure is a factor for characteristic deterioration.
To avoid the problem, in a proposed optical semiconductor device 90 shown in FIG. 5, a space B is provided between a soft resin 91 and a hard resin 92. This space is effective to prevent a compressive stress to be imposed on an optical semiconductor 93 even if a volume expansion occurs in the soft resin 91 as an environmental temperature elevates.
In the above example, the light emitted from (or coming into) the optical semiconductor chip 93 always passes through the space B, that is, air that has a lower refractive index compared to the resin. In this case, reflection occurred at an interface between the resin and the air causes a loss in amount of light, resulting in an output reduction of about 25–35%. Such the output reduction causes a problem associate with deterioration of the performance as the optical semiconductor device.
As for the soft resin 91 and the holder 94, a difference in expansion coefficient between respective materials also causes a problem because it forms peeled or cracked parts in the product.
The holder 94 commonly employs a lead-frame-insert injection-molded product in its structure. In this molding method, strictly, an air layer is interposed between the frame and the insert-molded product. The air layer expands under a high-temperature ambient and applies pressure to the soft resin, which produces bubbles therein. The production of bubbles causes a problem because it lowers the optical output similar to the above case and deteriorates the performance as the optical semiconductor device.